In the past, technique for detecting the pupil of a subject has been known. This technique is applicable to detection of an inattentive driving, detection of drowsiness of a driver, investigation of the degree of interest of a product, data input to a computer, and the like.
With respect to such pupil detection technique, the following Patent Literature 1 discloses a method of detecting a pupil based on a difference between a bright pupil image and a dark pupil image. The method detects the amount of difference in corneal reflection position between the two images imaging a bright pupil and a dark pupil and performs position correction in which one image is moved by an amount corresponding to the amount of difference in corneal reflection position to match the pupil portions of the both images substantially. Furthermore, the difference between the both images is obtained.